A sitemap is an XML file enumerating the URLs on a site, used by search engines to discover, prioritize, and schedule crawl of those pages.
The standard sitemap protocol (sitemaps.org) supports up to 50,000 URLs and 50MB per file; larger sites use a sitemap index referencing multiple sub-sitemaps. Each URL entry can include <lastmod> (last modification date), <changefreq> (hint), and <priority> (hint). Specialized sitemap extensions exist for images (<image:image>), video (<video:video>), and news (<news:news> for content under 2 days old).
For ecommerce, common sitemap structures: a sitemap index at /sitemap.xml pointing at /sitemaps/products.xml, /sitemaps/categories.xml, /sitemaps/brands.xml, /sitemaps/pages.xml, /sitemaps/blog.xml, and /sitemaps/images.xml. Each sub-sitemap paginated if it exceeds the 50k URL limit.
Sitemaps are submitted via Google Search Console, Bing Webmaster Tools, and Yandex Webmaster. Sitemap submission does not guarantee crawl or indexing — Google uses the sitemap as one of many signals.